1. Field of the Invention
The invention relates to a powdery material for an active material which is the main material of a secondary cell using an alkali as the electrolyte, electrode members such as a negative electrode and positive electrode and a secondary cell. More particularly, the invention relates to a powdery material of a hydrogen-storing compound (hydrogen storage compound) for use as the main material of a negative electrode, a negative electrode formed of the hydrogen-storing compound powdery material, a powdery material comprised of nickel hydroxide subjected to coating treatment for use as the main material of a positive electrode, a positive electrode formed of the nickel hydroxide, a secondary cell comprising the above negative electrode and/or the above positive electrode and a method for manufacturing these.
2. Related Background Art
Recently, since the amount of CO.sub.2 gas contained in the atmosphere increases, the possibility of global warming to occur due to the greenhouse effect has been pointed out. For example, in a thermal power plant, thermal energy obtained by burning of fossil fuel and the like is converted into electric energy, but the construction of a new thermal power plant has become difficult because a great amount of CO.sub.2 gas is discharged by burning. Thus, as an effective utilization of electric power produced by generators of a thermal power plant or the like, a so-called load levelling, levelling of loads by storing the night power as surplus power into secondary cells installed at general houses and using it at the daytime during which the power consumption is large, is proposed.
For an electric automobile which discharges no substance containing CO.sub.x, NO.sub.x, hydrocarbons or the like associated with air pollution, the development of secondary cells having an indispensable high energy density is expected. Furthermore, in uses for power sources of portable equipments such as book-type personal computers, word processors, video cameras and portable telephones, the development of a small-sized, light-weight and high-performance secondary cell is an urgent need.
Under such circumstances as mentioned above, a high-capacity secondary cell using a hydrogen-storing alloy storing hydrogen at a high density for the negative electrode of an alkali secondary cell with an alkali solution employed as the electrolyte, what is called nickel-metal hydride battery (cell) (hereinafter, referred to as nickel-metal hydride battery) has been put into practical use.
As hydrogen-storing alloys for the negative electrode of a hydrogen-storing alloy, Mischmetal alloys typified by Mm(Ni--Co--Mn--Al).sub.5, transition metal alloys typified by Ti--Zr--Ni--V--Cr--Co--Mn and magnesium-nickel alloys such as Mg.sub.2 Ni and MgNi have been researched and the Mischmetal alloys and the transition metal alloys are put into practical use as electrode materials.
Both the Mischmetal alloys and the transition metal alloys have a practical capacity lower than the theoretical capacity and are desired to be further improved. Besides, as compared with the nickel-cadmium battery using a cadmium negative electrode, they have had a problem of being subject to oxidization and deterioration during the overcharge. In this regard, a method of coating hydrogen-storing alloy powder with an oxidation-resistant metal is proposed in Japanese Patent Application Laid-Open Nos. 61-64069 and 61-101957, but no secondary cell sufficiently more resistant to overcharge than the nickel-cadmium battery has yet been obtained.
With respect to the magnesium-nickel alloys, it was made public in the 37-th Battery Symposium in Japan, p. 389 (1996) that as high discharge capacity as 750 mAh/g at the first time of the charge-discharge cycle was obtained in an electrode using a Mg.sub.2 Ni alloy powder prepared by the mechanical grinding method, but the discharge capacity decreased with the progress of the charge-discharge cycle and no magnesium-nickel alloy electrode having a stable high discharge capacity has yet been obtained.
In the nickel-metal hydride battery, nickel hydroxide is employed as a positive electrode active material as is the case of the positive electrodes of other alkali secondary cells such as the nickel-cadmium battery. Since this nickel hydroxide is low in conductivity, cobalt monoxide or cobalt hydroxide are added in forming the positive electrode in addition to nickel hydroxide to increase the conductivity, thereby enhancing the utilizing efficiency of the positive electrode active material. However, there has remained a problem of low utilizing efficiency of the positive electrode active material of the battery at the initial stage of the charge-discharge cycle.